H. R. Hart

Comparison of linear and circular polarization for magnetic resonance imaging

Abstract A comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented. Simulation images in good agreement with the experimental scans are described. The simulations are calculated with a model in which a homogeneous, isotropic cylinder of lossy dielectric material and infinite axial extent is immersed in a uniform rf magnetic field perpendicular to the axis. It is found that with the usual linear polarization, reconstructions of uniform objects have regions of decreased intensity. These artifacts are shown to arise from dielectric standing wave effects and eddy currents. The effects become more severe as the frequency or object size is increased, and depend upon the complex conductivity of the object. Results indicate that a significant reduction in the artifact intensity is achieved when circular polarization is employed for both transmission and reception. The expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√2) have been realized in practice with cylindrical objects and human subjects.

3D reconstruction of the brain from magnetic resonance images using a connectivity algorithm

We present high resolution three dimensional (3D) connectivity, surface construction and display algorithms that detect, extract, and display the surface of a brain from contiguous magnetic resonance (MR) images. The algorithms identify the external brain surface and create a 3D image, showing the fissures and surface convolutions of the cerebral hemispheres, cerebellum, and brain stem. Images produced by these algorithms also show the morphology of other soft tissue boundaries such as the cerebral ventricular system and the skin of the patient. For the purposes of 3D reconstruction, our experiments show that T1 weighted images give better contrast between the surface of the brain and the cerebral spinal fluid than T2 weighted images. 3D reconstruction of MR data provides a non-invasive procedure for examination of the brain surface and other anatomical features.

Signal, noise, and contrast in nuclear magnetic resonance (NMR) imaging

Calculations of the sensitivity of the saturation recovery and inversion recovery pulse sequences used in nuclear magnetic resonance imaging show the former to be superior in discriminating between tissues with the same proton density but different T1‘s. Two other pulse sequences, which are combinations of the above, have also been analyzed. These have lower T1 discrimination sensitivity, but other considerations, such as self-normalization, may still make them attractive. The calculations are only valid for selective excitation pulse sequences in which the selected slice profiles are approximately rectangular, and thus a sin(bt)/t radio frequency excitation is desirable. In order to ensure that the saturation recovery sequence gives valid results for pulse repetition times comparable to or shorter than T2 it is necessary to destroy the coherence between pulse applications. For this purpose we use a series of “spoiler” gradient pulses between pulse trains. The saturation recovery pulse sequence also has the advantage that, by the correct choice of interpulse spacing, sensitivity close to the optimum T1 discrimination can be achieved over a wide range of T1 values. This has the potential advantage to the clinician of simplifying his choice of parameters for imaging.

Magnetic resonance imaging of the temporomandibular joint meniscus

This report describes early experience with magnetic resonance imaging (MRI) of the temporomandibular joint meniscus in which surface coil technology was used. The results suggest remarkable imaging capabilities and speed with noninvasive methods.

Magnetic and Mössbauer spectroscopic characterization of coal

We sketch a role that solid state science, especially magnetic and Mossbauer spectroscopic characterization, can play in understanding coal, with its impurities. The Mossbauer characterization has potential impact on essentially all precombustion coal clean‐up procedures. We review concurrent Mossbauer studies on coal in order to demonstrate that this technique has much to offer as an improved analytical method for determining forms of sulfur in coal. Our magnetization analysis throws light on the magnetic properties of coal impurities, ash and pyrite, which facilitate magnetic separation clean‐up. It is demonstrated that simple crushing of coal can be responsible for transformations which aid in the separation. The two methods of analysis are shown to be useful for understanding coal liquefaction residues from the point of view of their removal magnetically.

Very Heavy Solar Cosmic Rays: Energy Spectrum and Implications for Lunar Erosion

The energy spectrum of solar cosmic-ray particles of the iron group has been determined for the first time over the energy range from 1 to 100 million electron volts per nucleon by the use of glass removed from the Surveyor 3 spacecraft. The difference between the observed (energy)-3 spectrum and the limiting spectrum derived previously from tracks in lunar rocks gives an erosion rate of 0 to 2 angstroms per year. High-energy fission of lead, induced by galactic cosmicray protons and alpha particles, has also been observed.

EFFECT OF FAST‐NEUTRON IRRADIATION ON MAGNETIC PROPERTIES AND CRITICAL TEMPERATURE OF SOME TYPE II SUPERCONDUCTORS

Experimental results are given for fast-neutron irradiation effects on niobium, Nb --Ta, Nb/sub 3/Sn, Nb/sub 3/Al, V/sub 3/Ga, and V/sub 3/Si. (R.E.U.)

Effect of Thermal‐Neutron Irradiation on the Superconducting Properties of Nb3Al and V3Si Doped with Fissionable Impurities

The superconducting critical temperatures and current carrying capacities of Nb3Al and V3Si doped with small amounts of uranium and boron and subsequently irradiated with thermal neutrons are reported. While the critical temperatures are substantially unaffected, the critical current densities are dramatically increased by the internal fission of uranium in both materials. Samples of Nb3Al containing 0.321 at.% uranium and V3Si containing 0.19 at.% uranium irradiated with 1.7×1018 neutrons/cm2 give superconducting critical current densities over 106 A/cm2 at 30 kOe. The boron‐doped samples, however, showed no effects arising from fission of boron. Two new experimental techniques have been used in this investigation. First, uranium and boron analyses as well as information on homogeneity were determined by use of the Price‐Walker nuclear track detectors. Secondly, the critical current measurements were made on 50‐mg samples of 70‐μ powders by a technique that measures the generation of odd‐harmonic voltage...

Orbital Magnetic Resonance Imaging

Magnetic resonance images of the eye and orbit performed with surface coils at 1.5 tesla showed anatomic details superior to those of conventional third- and fourth-generation computed tomography.

Cosmic-Ray Tracks in Plastics: The Apollo Helmet Dosimetry Experiment

Counts of tracks from heavy cosmic-ray nuclei in helmets from Apollo missions 8 and 12 show variations caused by solar modulation of the galactic cosmic-ray flux. Specific estimates of the biological damage to certain nonreplaceable cells by track-forming particles during these space missions indicate that the fraction of deactivated cells could range from a lower limit of 3 x 10-7 to an upper limit of 1.4 x 10-4.